A planar waveguide laser amplifier is configured with a waveguide element including a laser medium and clad layers which have a refractive index smaller than that of the laser medium and are put on the top and bottom faces thereof. In the waveguide element, a laser beam is totally reflected between a core layer and clad layer to propagate in a waveguide mode. In a planar waveguide element, population inversion is produced by excitation light, and the laser beam is amplified by passing through the planar waveguide element.
When a laser beam is inputted to the planar waveguide laser amplifier, the laser beam is inputted through an anti-reflection film formed in a part of the planar waveguide element. The amplification gain of the laser beam is improved by making the laser beam reflect multiple times on total reflection films formed on the planar waveguide element's side faces opposing with each other to make the beam path length longer. The amplified laser beam is outputted through an anti-reflection film formed on a part of the planar waveguide element. In Patent Document below, the configuration of this planar waveguide laser amplifier is described.